1. Field of the Invention
The present invention relates to a hydraulic control system of a four-speed automatic transmission for an automotive vehicle, and in particular to a hydraulic control system of an automatic transmission in which second-to-fourth skip shifting or fourth-to-second skip shifting is possible, line pressure is controlled variably to improve the power train efficiency, and shift feeling is improved when shifting from a neutral mode to a drive mode or a reverse mode.
2. Description of the Prior Art
A conventional vehicle automatic transmission has a torque converter and a multiple stage transmission gear mechanism connected with the torque converter. The gear mechanism includes hydraulically applied friction members for selecting one of the gear stages of the transmission gear mechanism in accordance with vehicle operating conditions. Control valves direct fluid pressure to the friction members. A hydraulic pump produces pressure required to operate the friction members and control valves.
A commonly used vehicle automatic transmission has a hydraulic torque converter which generally includes a pump impeller, a turbine runner with an output shaft member, and a stator disposed between the pump impeller and the turbine runner. The pump impeller is connected with and driven by an engine output shaft. Fluid is circulated by the engine driven pump impeller through the turbine runner with the aid of the stator. The stator deflects the fluid from the turbine runner to a direction where the fluid flow does not disturb the rotation of the pump impeller when the fluid flows into the pump impeller.
An automatic shift is performed by operating the friction members, such as clutches or a kickdown brake, at each shift change. A manual valve is designed to be fed with fluid from a hydraulic pump and to feed the fluid to a shift control valve. Ports of the manual valve are selectively opened or closed by selection of a position of a selector lever.
In the case of a 4-speed automatic transmission, the shift control valve is operated by an electronic control system. In a conventional automatic transmission control system, because hydraulic pressure is fed in two modes, a drive range and a reverse range, it is difficult to improve the pump efficiency and the power train efficiency. Furthermore, it is extremely difficult to make a skip shifting which results in a slow shifting response.